A 32-year-old Rh-negative primigravida at 34 weeks gestation presents with vaginal bleeding (500 mL). She is unsensitised (negative indirect Coombs test). Regarding management of Rh isoimmunisation risk in this case, all of the following are appropriate EXCEPT:

Explanation

## Management of Sensitising Event: Antepartum Haemorrhage in Rh-Negative Woman ### Clinical Context A 34-week Rh-negative, unsensitised woman with 500 mL vaginal bleeding faces risk of fetomaternal haemorrhage (FMH) and Rh sensitisation. The standard approach is: 1. Quantify FMH 2. Calculate anti-D dose 3. Administer within 72 hours 4. Perform post-delivery serology ### Quantification of Fetomaternal Haemorrhage **Key Point:** Kleihauer-Betke test (or flow cytometry) is **mandatory** to: - Count the number of fetal red cells in maternal blood - Calculate the precise dose of anti-D required - Avoid under-dosing or over-dosing ### Anti-D Dosing Calculation **High-Yield:** The standard dose formula is: $$\text{Anti-D dose (IU)} = \text{Volume of fetal RBCs (mL)} \times 500 \text{ IU/mL}$$ Alternatively: **1 mL of fetal red cells = 500 IU anti-D** **Example calculation:** - If Kleihauer-Betke shows **10 mL of fetal red cells**, the required dose is: $$10 \text{ mL} \times 500 \text{ IU/mL} = 5000 \text{ IU (10 vials of standard 500 IU)}$$ - A **single standard dose of 500 IU is NOT sufficient**—this would only cover 1 mL of fetal red cells. **Warning:** The statement "a single standard dose of 500 IU is sufficient" is a **trap answer**. It confuses the standard dose used for routine prophylaxis (at 28 weeks, when FMH is minimal) with the calculated dose needed after a significant sensitising event. After 500 mL vaginal bleeding with 10 mL fetal red cells, **5000 IU (10 standard vials) is required**. ### Post-Delivery Serology **Clinical Pearl:** A repeat indirect Coombs test (antibody screen) should be performed **3–4 months postpartum** to: - Confirm that sensitisation did not occur despite anti-D prophylaxis - Detect any delayed sensitisation - Provide baseline for future pregnancies This is standard practice in all Rh-negative women who receive anti-D. ### Summary: Anti-D Dosing in Sensitising Events | Scenario | FMH Volume | Anti-D Dose Calculation | Actual Dose | |----------|------------|------------------------|-------------| | Routine prophylaxis (28 weeks) | Minimal (~0.4 mL) | Standard dose | 500 IU | | Small antepartum bleed | 1–2 mL | 500–1000 IU | 1–2 vials | | Moderate bleed (10 mL RBCs) | 10 mL | 10 × 500 = 5000 IU | **10 vials** | | Large bleed (20 mL RBCs) | 20 mL | 20 × 500 = 10,000 IU | **20 vials** | **Mnemonic:** **FMH-500** = Fetal red cells (mL) × 500 IU = total anti-D needed.